The present invention relates, in general, to the field of bio-medical telemetry, and more particularly to a sensor-transmitter for use with human patients under clinical or ambulatory observation.
The detection and measurement of physiological data has become a highly developed art in recent years, and many systems have been designed for this purpose. Such systems have included sensors of various types which may be secured to a patient to detect various phenomena and which produce electrical signals carried by means of electrical leads to suitable measuring and displaying equipment. However, for many purposes, the requirement for a cable connection between a measuring device and a sensor is undesirable, for it not only is uncomfortable to the patient, but it substantially restricts his freedom of movement. Accordingly, telemetry systems utilizing a small, self-contained transmitter in combination with the sensor have been used to transmit the sensed data to a remote receiver. Such systems are described, for example at pages 148 to 160 of "Bio-Medical Telemetry," by R. Stewart MacKay, (Second Edition, Wiley, 1968). Typically, the transmitters of such systems utilize a suitable blocking oscillator having a resonant circuit, the winding of which serves the dual function of tuning the oscillator and generating a radio frequency field which may be detected by the antenna of a receiver. The oscillator is modulated by the sensor to emit bursts or pulses of radio frequency energy at a rate that corresponds to the magnitude of the parameter being measured.
Sensor-transmitters of this general type are very advantageous were remote measurements of parameters are required or desirable. However, numerous difficulties have been encountered in the past in the use of such transmitters, for they have often been inaccurate, unreliable in operation, and difficult to manufacture. One common difficulty resides in the fact that the antenna winding used to transmit the radio frequency, and thus the resonant circuit of which it is a part, may be loaded electrically by an external object, which loading may vary from time to time in accordance with the nature and location of the external object. Such loading affects the burst rate of the transmitter oscillator, so that the output does not always bear a fixed relation to the magnitude of the phenomenon under investigation, but will depend in a variable manner upon the presence and nature of such external objects. Such a system can produce erroneous, or at least unreliable or misleading, indications of the phenomenon under investigation. This is particularly a serious problem where physiological data is being monitored, for human tissue can produce such a loading effect, and this can result in an erroneous reading of, for example, the temperature of a patient undergoing medical observation, if the device has not been carefully calibrated to take into consideration the loading effect created when the transmitter is secured to the skin of the patient.
Additional problems are encountered in the use of prior sensor-transmitters in that such devices are generally susceptible to considerable interference in the radio-frequency bands, making it difficult to obtain accurate readings over a short period of time without resorting to complex coding systems and the like. An additional problem in such devices has been that of obtaining a fast response to a temperature measurement, while preserving the stability of operation of the device and maintaining an accurate measure of the parameter of interest. Since small self-contained transmitters of this type are generally battery-powered, another problem that has been encountered is the provision of a circuit that will produce a reliable output over the period of life of the battery.
In the prior copending applications, referenced above, a small, self-contained physiological transmitter assembly has been described which overcomes many of the difficulties of prior art systems; however, the device of those applications was designed for a one time use, for the battery power supply was built as an integral part of the device. Although it has now been found that a single-use device of this type is not sufficiently economical, the requirements for accuracy of measurement in a mass-produced item could not be met in any other way at the time of that application.
One of the prime requirements for a sensor-transmitter of this type is that it be small and light in weight, so that it will be comfortable and safe to wear. This requires a very compact circuit arrangement and in order to insure that the frequency of operation of the device remains within a predetermined range and that each sensor-transmitter produces the same output characteristics, the arrangement of elements within the housing becomes critical to an accurate measurement of the parameter being monitored. Further, the placement of elements must be easily reproducible, so that the unit can be mass-produced while still obtaining identical data from each one with a minimum of calibration. In addition, to reduce the cost of the unit, the circuit arrangement must be such that it may easily be assembled and can be adapted for automated assembly. Further, care must be taken to insure that the transmitted signal itself does not interfere with the sensor, and thus affect the accuracy of the device.